1. Field of the Invention
This invention relates to a magnetic toner for developing an electrostatic image, used in image forming processes such as electrophotography and electrostatic recording, to render electrostatic latent images visible.
2. Related Background Art
As electrophotography, various methods are disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 42-23910 and Japanese Patent Publication No. 43-24748 and so forth.
Developing systems applied in such electrophotography are roughly grouped into a dry developing method and a wet developing method. The former is further grouped into a method making use of one-component developers and a method making use of two-component developers. The developing method making use of one-component developer has a feature that developing apparatus can be made small-sized. This method, however, has difficulty in imparting sufficient triboelectricity to the toner and hence it has the problem that the allowable scope for designing toners and developing systems is narrow. On the other hand, the developing method making use of the two-component developer can impart sufficient charges to toners and hence has the advantage that it has wider tolerance for designing, but has a problem that it requires a means for uniformly controlling the mixing ratio of the toner and the carrier, making its apparatus complicated.
As toners used in these developing methods, fine powders comprising a colorant such as a dye or pigment dispersed in a natural or synthetic resin are hitherto used. For example, toner particles are prepared by pulverizing a dispersion of a colorant in a binder resin such as polystyrene to a size of about 1 to 30 .mu.m. As a magnetic toner, toner particles containing magnetic material particles such as magnetite are used.
Toners have positive charges or negative charges depending on the polarity of electrostatic latent images to be developed. In order to charge toners, it is possible to utilize triboelectric chargeability of resins that compose toners. In such a method, however, the chargeability of the toner is so small that toner images obtained by development tend to be foggy and unclear. In order to impart a desired triboelectric chargeability to toners, a dye or pigment capable of controlling chargeability and also a charge control agent are commonly added.
However, toners containing such charge control agents tend to contaminate the toner carrying members such as the developing sleeve, and hence such toners tend to cause a decrease in quantity of triboelectricity as the number of copies taken increases, resulting in a decrease in image density. Charge control agents of a certain type have a small quantity of triboelectricity and tend to be affected by temperature and humidity, and hence may cause variations of image density in accordance with environmental changes. Certain charge control agents have a poor dispersibility in resins, and hence toners making use of such charge control agents tend to have uneven triboelectricity between toner particles, tending to cause fogging. Certain charge control agents have poor storage stability so that toners may undergo a decrease in triboelectric performance during long-term storage.
As a means for solving these problems, Japanese Patent Publication Nos. 43-17955, 55-42752 and 63-1994 propose various kinds of metal complexes as charge control agents. These charge control agents certainly have a good negative triboelectric chargeability. Most of them, however, are chromium compounds, and more improvement has been sought from the viewpoint of environmental safety.
Japanese Patent Application Laid-open Nos. 61-155464, 61-101558 and 61-155463 propose iron complexes.
These publications disclose that the iron complexes have a negative triboelectric chargeability and have a very good compatibility with resins. However, studies made by the present inventors have revealed that only some of them can provide magnetic toners providing a more stable image quality in the one-component development system as described later.
In order to maintain the high image quality obtained at the initial stage, without regard to the number of copies taken, it is insufficient to only maintain the quantity of triboelectricity. The particle size distribution of the toner at the initial stage must also be kept constant. In particular, it is important for the toner particles of relatively large particle size (coarse powder) to be used in development in a good efficiency to prevent their accumulation. For such purpose, the magnetic properties and quantity of triboelectricity of magnetic toners must be adjusted to proper values. Taking these points into account, the present inventors have studied charge control agents to find but the quantity of negative triboelectricity becomes smaller when organic ammonium ions are used as counter ions. The reason is unclear, but it is presumed to be due to a positive triboelectric chargeability inherent in organic ammonium ions as generally known in the art. Japanese Patent Application Laid-open No. 61-101558 discloses that organic ammonium ions are effective to improve the dispersibility of metal complexes in resins. According to the studies made by the present inventors, however, in the case of one-component developers making use of magnetic toners, the organic ammonium ions exert greater influence on a decrease in triboelectric chargeability than on the improvement of dispersibility, so that the coarse powder in the toner accumulates as developing is repeated many times, to cause a slight lowering of image quality.
Polyvalent inorganic ions disclosed in Japanese Patent Application Laid-open No. 63-267793 also have caused accumulation of the coarse powder in toners. Negative charge control agents disclosed in Japanese Patent Application Laid-open No. 63-267793 have polyvalent ions as counter ions to make the molecular structure larger, so that they show more improved dispersibility in resins than the negative charge control agent disclosed in Japanese Patent Application Laid-open No. 61-155464. As a result, the carrier contamination due to the toner can be repressed prolonging the life time of the developer from 50,000 to 100,000 sheets copying to 200,000 sheet or more as so disclosed therein. According to the studies made by the present inventors, however, in order to maintain the good image quality at the initial stage using a magnetic toner in one-component development, it is necessary not only to keep the quantity of triboelectricity constant, but as previously stated, also to maintain a high quantity of triboelectricity, so that the coarse powder in the toner can also participate in the development. From such viewpoints, the iron complexes of polyvalent ions as disclosed in Japanese Patent Application Laid-open No. 63-267793 are not suited for magnetic toners. The coarse powder tends to accumulate also in the case of the iron complexes having a substituent such as a nitro group as shown in Japanese Patent Application Laid-open No. 61-155463, or those having a sulfonamide group, in Japanese Patent Application Laid-open No. 61-155464.
Meanwhile, with regard to magnetic properties of magnetic toners, proposals are made as follows:
Japanese Patent Application Laid-open Nos. 58-95748, 58-98744 and 3-95578 report the magnetic properties of magnetic toners.
According to Japanese Patent Application Laid-open No. 58-95748, saturation magnetization has an influence on transport performance of magnetic toner particles. Those with a saturation magnetization less than 23 emu/g weaken magnetic transport power to tend to cause uneven development. Those with a saturation magnetization more than 50 emu/g require a large quantity of magnetic powder in magnetic toners to make fixing performance low or developing performance poor. Toner particles with a coercive force less than 150 oersted greatly lower the developing performance, and those with a coercive force more than 350 oersted strengthen agglomeration force of toner particles to cause a problem in toner transport performance.
Japanese Patent Application Laid-open No. 58-98744 discloses that coercive force of 150 oersted or more is required in order to obtain fog-free images in reversal development.
Japanese Patent Application Laid-open No. 3-95578 discloses to reduce the quantity of a magnetic material so that a color toner with less turbidity can be obtained. For this reason, the magnetic toner is made to have a saturation magnetization of 40 emu/g or less, and a magnetic roller (a developing sleeve) is designed so as to compensate for any lowering of the transport power of the magnetic toner. In any case, the saturation magnetization is controlled taking into account the transport performance of magnetic toners and the coercive force is controlled for developing performance. Although the image quality at the initial stage can be improved by controlling magnetic properties of the magnetic toner, it is difficult to control image deterioration due to changes in toner particle size that may occur as developing is repeated many times. In order to prevent the magnetic toner from changing particle size, both the quantity of triboelectricity of the magnetic toner and the magnetic properties thereof must be taken into account.